Method of producing alkali metal pyroarsenates



Patented Dec. 11, 1934 UNITED STATES PATENT OFFEQE IVIETHOD 0F PRODUCING ALKALI METAL PYROARSENATES ration of Pennsylvania No Drawing. Application January 4, 1934 Serial No. 705,280

'7 Claims.

This invention relates to the production of pyroarsenates and it is particularly concerned with the production of alkali metal pyroarsenates of high purity.

The alkali metal pyroarsenates exist only in the dry state, being hydrolyzed in the presence of water to the dialkali orthoarsenate. They are useful because of the high percentage of available arsenic oxide contained in them. l-Ieretofore it has been economically impossible to produce alkali metal pyroarsenates, of suiiicient purity to permit their commercial use as such. Impure pyroarsenates have been produced incidentally as an intermediate product in the methods of producing dialkali orthoarsenates by calcining together arsenious oxide and an alkali metal salt in the presence of an oxidizing agent and subse quently leaching the product with water. The sinter obtained in this process contains pyroarsenates associated with relatively large amounts of impurities, particularly uncombinecl reactants, and the subsequent leaching step serves the double function of removing the impurities and hydrolyzing the pyroarsenate to the dialkali orthoarsenate. Pyroarsenates in the pure state may be produced by calcining the pure dialkali orthoarsenateat elevated temperatures. This method is not, however, a commercially practical one.

It is the primary object of this invention to provide an improved process for the production of alkali metal pyroarsenates of high purity. Another object of this invention is to provide a process for the direct production of the alkali metal pyroarsenates in the dry state substantially free from uncombined reactants. It is more particularly an object of this invention to provide an improved method for the production of pure sodium pyroarsenate.

It has been found that when arsenious oxide (AS203) is sintered with an alkali metal nitrate and the corresponding alkali metal carbonate in suitable proportions to drive ofi all of the nitrogen present as nitric oxide (NO) the reaction will proceed to completion, and that by rapidly heating such a mixture to a temperature above about the fusion point of the alkali metal nitrate-alka- 11 metal carbonate eutectic and maintaining such temperature for a brief period, a product containing as high as 99.75% alkali metal pyroarsenate may be produced.

In the methods heretofore described for the production of dialkali orthoarsenate, which involved the step of heating together arsenious oxide, an alkali metal nitrate and an alkali metal carbonate, the respective compounds have been mixed in such proportions that the reaction taking place does not combine all of the alkali metal and the arsenious oxide present in the form of an alkali metal pyroarsenate and does not proceed to completion. The reaction product generally contains less than about 92% of sodium pyroarsenate. However, when arsenious oxide, sodium nitrate and sodium carbonate are mixed according to our invention in the proportions of '7 parts by weight arsenious oxide, 4 parts by weight sodiicn nitrate and 5 parts by weight sodium carbonate, and the raw mixture is rapidly raised to a temperature above sintering, preferably to a dull red heat, the reaction proceeds practically instantaneously according to the equation AszOs-l-iNdNOz-I-lNdzCOs:

3N2u4AS2O7-E- 4N0 4:092. Also, this reaction proceeds to completion and a product consisting of sodium pyroarsenate substantially free from uncombined reactants is obd tained. If desired, caustic soda or sodium bicarbonate in equivalent amounts may be substituted for the sodium carbonate described in the above reaction. Also, potassium pyroarsenate may be produced by the use of potassium salts for the corresponding sodium salts in the above mixture in equivalent amounts.

The purity of the alkali metal pyroarsenate obtained by the method of this invention will vary somewhat with the purity of the raw materials used and with the accuracy of their proportioning in the reaction mixture. The reactants are generally available commercially in quite pure form and by the use of these commercial raw materials and controlling the accuracy of the weights of the respective ingredients used to a variance from the theoretical of not more than 2% it has been found possible to obtain alkali metal pyroarsenates of 97 to 99.5% purity.

In carrying out the method of this invention the raw materials, preferably in finely divided or powdered form, are first intimately mixed in the proper proportions, for example in a drum mixer, to produce a uniform reaction mass. The reaction mass is then heated in a suitable furnace above its sintering temperature. The heating is preferably carried out in such a manner that the Whole of the reaction mass is raised to the desired temperature as rapidly as possible in order to prevent the sublimation of any substantial amount of arsenious oxide from the mix, thus preventing an unbalancing of the reaction mixture. It is sometimes desirable to incorporate in the reaction mixture prior to heating a slight excess of arsenious oxide to compensate for any loss by sublimation while heating to the reaction temperature. After the temperature of sintering is reached there appears to be less tendency for the arsenious oxide to sublime. For the purpose of heating the reaction mixture in the desired manner the method which has been found preferable is to place the reaction mixture in shallow pans in a tunnel furnace and to heat it by gas flames playing directly on the bottom of the pan. While the reaction will proceed at substantially any temperature above the sintering temperature of the reaction mass, it has been found most desirable to carry out the method at a dull red heat (586 to 600 C.). At this temperature the reaction takes place practically instantaneously and prolonged heating is unnecessary. Heating above this temperature is generally undesirable since no beneficial results are obtained and higher temperatures deleteriously affect the equipment. The product of this method is a sinter or cake of substantially pure alkali metal pyroarsenate which requires only grinding to be ready for use.

The following are typical of the results obtainable by the method of this invention in the production of sodium pyroarsenate: 350 pounds of finely divided arsenious oxide of 99.5% purity, 200 pounds of pure sodium nitrate, and 250 pounds of soda ash containing 99.5% sodium carbonate were intimately mixed in a drum mixer. After mixing, the raw mix was transferred to shallow iron pans and distributed in layers about one inch deep. These pans were heated in a tunnel furnace as rapidly as possible to a dull red heat. The heating was continued for about minutes, this being a convenient time schedule for charging and discharging the pans from the furnace and insuring sufficient time for the whole of the reaction mass to reach the desired temperature. The product which was obtained in cake form contained 99% sodium pyroarsenate.

Havin now particularly described this invention, what we claim is:

l. A method of producing pure alkali metal pyroarsenate comprising intimately mixing arsenious oxide, an alkali metal nitrate and the cor responding alkali metal carbonate in suitable proportions to liberate all of the nitrogen of the alkali metal nitrate as nitric oxide, and sintering said mixture at a temperature above the fusion point of the alkali metal nitrate-alkali metal carbonate eutectic.

2. A method of producing pure alkali metal pyroarsenate comprising intimately mixing arserious oxide, an alkali metal nitrate and the corresponding alkali metal carbonate in suitable proportions to liberate all of the nitrogen of the alkali metal nitrate as nitric oxide and sintering said mixture at dull red heat.

3. A method of producing pure sodium pyroarsenate comprising intimately mixing arsenious oxide, sodium nitrate and sodium carbonate in suitable proportions to liberate all of the nitrogen of the sodium nitrate as nitric oxide, and sintering said mixture at a temperature above the fusion point of the sodium nitrate-sodium carbonate eutectic.

l. A method of producing pure sodium pyroarsenate comprising intimately mixing arsenious oxide, sodium nitrate and sodium carbonate in suitable proportions to liberate all of the nitrogen of the sodium nitrate as nitric oxide, and sintering said mixture at dull red heat.

5. A. method of producing pure sodium pyroarsenate comprising intimately mixing about 7 parts by weight arsenious oxide, 4 parts by weight sodium nitrate and 5 parts by weight sodium carbonate, and sintering said mixture at a temperature above the fusion point of the sodium nitrate-sodium carbonate eutectic.

6. A method of producing pure sodium pyroarsenate comprising intimately mixing about 7 parts by weight arsenious oxide, 4 parts by weight sodium nitrate and 5 parts by weight sodium carbonate, and sintering said mixture at null red heat.

'7. A method of producing pure sodium pyroarsenate comprising intimately mixing about '7 parts by weight arsenious oxide, 4 parts by weight sodium nitrate and 5 parts by weight sodium carbonate, and sintering said mixture at a temperature between about 500 and 600 degrees centrigrade.

GLENN H. WAGNER. WILBURN L. MOWE. 

